This invention relates to anodized aluminum and more particularly to low temperature, vapor sealing of anodized aluminum.
Anodized aluminum is usually sealed to render it impervious to elements which could adversely affect the aluminum substrate, because, in many instances, especially in architectural applications, it will be exposed to the atmosphere for many years. It is therefore imperative that the seal be of very high quality to ensure satisfactory service.
Since anodized aluminum normally requires treatment with a sealing solution at or near its boiling point to produce a high quality seal, considerable effort has been expended to develop methods of sealing at a lower temperature. Sealing at lower temperatures is desirable since it would aid in conserving energy resources by reducing the energy required to produce a high quality seal.
High quality seal, as used herein, is defined as a sealed, smudge-free anodized aluminum, which, after being immersed for a period of 15 minutes in a solution containing 2.0 wt. % chromic and 3.5 wt. % phosphoric acid at a temperature of 100.degree. F, has a weight loss of not more than 2 mg./in..sup.2. This seal quality evaluation is known to those skilled in the art as the "acid dissolution test".
Sealed anodized aluminum is resistant to staining. Thus, sealing quality can be determined qualitatively by a dye stain test known as "Standard Method for Measurement of Stain Resistance of Anodic Coatings on Aluminum" (ASTM B136-72). In this method, after conditioning the sealed, anodized surface with a nitric acid treatment, a dye test solution is placed thereon for a period of about 5 minutes, then this test area is washed with water and rubbed with pumice powder. Staining of the anodized finish after this treatment indicates a poorly sealed anodic coating. Conversely, absence of stain indicates a satisfactory seal.
In the prior art, normally a satisfactory seal was produced on anodized aluminum by treating it with an aqueous solution at or near the solution boiling point. The aqueous solution normally comprised distilled or deionized water or a combination of such high purity water with a hydrolyzable metallic salt such as nickel acetate or a substance which would enhance sealing. For example, my previous patent (U.S. Pat. No. 3,791,940) discloses sealing undyed aluminum using metallic salts such as nickel acetate in an aqueous solution at about 212.degree. F or boiling. As another example of materials that were added to high purity water, Wallace U.S. Pat. No. 3,822,156, discloses sealing anodized aluminum in a solution of triethanolamine in hot water, with optimum sealing temperatures being near the boiling range of the solution. Because of smudge formed during these sealing processes, it is taught in these patents that the sealed anodized aluminum is thereafter desmudged with mineral acid, thus requiring an additional step after the sealing operation to produce a smudge-free anodized finish on aluminum.
In the prior art, it is also taught that anodized aluminum can be sealed in a liquid at or close to room temperature (20.degree.-25.degree. C). Yoshimura et al disclose in "Effect of Room-temperature Sealer Containing Ferricyanide and Phosphate on Anodic Aluminum Oxide Film", Journal of Metal Finishing (Japan), Vol. 19, No. 12 (1968), p. 504 that anodized aluminum can be sealed at room temperature in an aqueous solution containing ferricyanide, phosphate, and calcium chloride at a pH of 4.5. This article, however also indicates that immersion of anodized aluminum in a 25% NH.sub.4 OH solution, pH of 11.3, does not produce a seal until the temperature of the solution reaches 80.degree. C.
Quite surprisingly, a method has been discovered that will produce a high quality, smudge-free seal on anodized aluminum utilizing ammonium hydroxide (NH.sub.4 OH) at ambient temperatures but with faster sealing rates being effected by raising the temperature slightly.